I believe this is your issue: Main._MODID+":"+item.getRegistryName() in registerItemModels.

getRegistryName also returns the domain as part of the string, so this line becomes "toybox:toybox:explmin". You've doubled up the domains. You can also see this in the screenshot, the resource thinks it belongs to "toybox:toybox:explmine#inventory". Remove The unnecessary "Main._MODID + ":" from your modelResourceLocation and you should be golden.